SENTRE and TREND attenuating systems

ARIZONA DEPARTMENT OF TRANSPORTATION
REPORT NUMBER: FHWA-AZ-8802 & FHWA-AZ-8803
SENTRE AND TREND
ATTENUATING SYSTEMS
Final Report
Prepared by:
Greg Rollins
Lany A. Smfield
Arizona Transportation Research Center
College of Engineering & Applied Sciences
Arizom State University
Tempe, Arizona 85287
July I991
Prepared for:
Arizona Department of Transportation
206 South 17th Avenue
Phoenk, Arkona 85007
in rooperation with
U.S. Depaltment of Transportation
Federal Highway Administration
The contents of this report reflect the views of the authors who are responsible for the facts and the
accuracy of the data presented herein. The contents do not necessarily reflect the official views or
policies of the Ariiona Department of Transportation or the Federal Highway Administration. This
report does not constaute a standard, specification, or regulation Trade or manufacture* names
which may appear herein are cited only because they are considered essential to the objectives of
the report. The U. S. Gwernment and the State of Arizona do not endorse products or
manufacturers.
Technical Report Documentation Pagi
I. Report No. 2. Government Accession No. 3. Rscipientb Catalog No.
MWA-AZ-8802 & FHWA-AZ-8803
I
1. Tide and Subtitle 5. Report Date
Julv 1991
SENTRE and TREND Attenuating Systems 6. Performing Organization Code
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7. Author (s) 8. Performing OrganIration Report No.
Greg Rollins and Lany A. Scofield
3. Performing Organizdon Name and Address 10. Work Unit No.
ARIZONA TRANSPORTATION RESEARCH CENTER
206 S. l7TH AVENUE, MAIL DROP 075R 11. Contract or &ant No.
PHOENIX, ARIZONA 85007 HPR-PL-l(37) ITEM 114
12. Sponsoring Agency Name and Address 13.Type of Report & Period Covered
ARIZONA DEPARTMENT OF TRANSPORTATION FINAL Dec. 1988 -Jan. 1991
206 S. 17TH AVENUE
PHOENIX, ARIZONA 85007 14. Sponsoring Agency Code
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15. Supplementary Notes
Prepared in cooperation with the U.S. Department of Transportation, Federal Highway Administration
The objective of this study was to construct SENTRE and TREND attenuator systems and evaluate their
in-sewice performance as set forth in NCHRP-230 for a period of two years. The evaluation was conducted
as part of ADOTs New Product Evaluation Program. ~ o tohf the systems had performed satisfactorily in full
scale crash testing conducted during other research efforts, and were designated as experimental by ihe
FHWA at the onset of the study.
Four SENTRE end treatments were installed, one downstream and one upstream on both sides of a
single bridge, as part of ADOT construction project HES-02202(33)P. Likewise, four TREND end treatments
were installed, one downstream and one upstream on both sides of a single bridge, as part of ADOT
construction project F-081-l(2). Both of these bridges extend over canals. The canals have parallel utility
roads, and access to these roads precludes the full development of the length-of-need (LON) required with
the use of guardrail. Following construction, the in-sewice performance was assessed from accident and
maintenance cost records.
The systems were constructed without major difficulty, the TREND was completed and operational
November 3,1988, and the SENTRE was operational December 22,1988. During the evaluation period the
SENTRE was hi twice and the TREND was hi once. None of the impacts lent themselves to a safety
performance evaluation of the attenuators, however, each of these hits required repair and replacement parts
for the attenuators.
The repair process was found to be simple and in each case was performed without difficulty. However,
the delivery time for replacement parts far exceeded EASl's claim of 48 hours. Delivery times ranged from
two weeks to 37 days. The ATRC has recommended stockpiling spare attenuator parts in proximity to the
attenuators for their timely repair. Further monitoring of the TREND is recommended until it is judged
operational by the FHWA.
Wenuator, End Treatment, Transition, End
rerminal, Attenuating System, SENTRE, TREND,
Zuardrail, Bridge Rail, Traffic Barrier,
4ppurtenance.
Unclassified I Unclassified I
18. Distribution Statement
Document is available to the U.S.
Public through the National
Technical Information Sewice,
Springfield, Virginia, 22161
TABLE OF CONTENTS
INTRODUCTION ...................................................................................................... 1
Problem Statement ..............................................................................................1..
ADOT Standard End Treatments. .....................................................................2.. ...
Traffic Barrier Performance Criteria ....................................................................3.. .
The SENTRE and TREND Systems .....................................................................4.
OBJECTIVES .........................................................................................................6..
In-Service Performance Evaluation ......................................................................6.
LOCATION OF PROJECTS ..................................................................................... 6
AZ-8802: SENTRE End Treatment Field Installation ............................................ 6
AZ-8803: TREND End Treatment Field Installation .............................................7.
CONSTRUCTION ...................................................................................................8..
Procedure ...........................................................................................................8.. .
ECONOMICS .......................................................................................................... 14
Construction Cost Comparison ............................................................................ 14
Maintenance Costs .............................................................................................. 14
INCIDENTS ..........................................................................................................1..5
The SENTRE System .......................................................................................... 15
The TREND System ............................................................................................1 8
CONCLUSIONS ..................................................................................................2..0.
Construction ......................................................................................................2..0
In-Service Performance ....................................................................................... 21
Future Considerations .......................................................................................2..1
RECOMMENDATIONS. ........................................................................................2..2
REFERENCES .....................................................................................................2..3
APPENDIX A - FHWA Approved Workplan .........................................................A..1.
APPENDIX B Current ADOT Standard Details ...................................................B..1.
APPENDIX C - EASl Technical Discussion ........................................................C....l
APPENDIX D - Construction Plans for AZ-8802 (SENTRE) ..................................D. l
APPENDIX E - Construction Plans for AZ-8803 (TREND) .....................................E. l
LlST OF FIGURES
. .
1 .C anal and Utlllty Road ......................................................................................1. ..
2 .B ridge With Full LON .......................................................................................2..
3 .A Breakaway Cable Terminal (BCT) ....................................................................2
4 . An ADOT Standard Attenuator Assembly ..................................................... 3 5 .S ENTRE End Treatment ...................................................................................5..
6 .T REND End Treatment ......................................................................................5..
7 . AZ-8802: SENTRE End Treatment Installation Site Prior to Construction .......... 7
8 .A Z-8803: TREND End Treatment Installation Site Prior to Construction ............7
9 . Assembly of SENTRE and TREND Post 2 ...................................................... 9
10 .B ase Plate Assembly .......................................................................................9..
1 1 .P ost Attached To Base Plate. ..........................................................................1 0
12 .F ront Anchor of Redirecting Cable ................................................................1..1
13 .R ear Anchor of Redirecting Cable .................................................................1..1
14 .S pecial Downstream Splice ............................................................................. 1 1
15 . Proper Splice ................................................................................................... 12
16 . TREND Attached to Front of Concrete Parapet Wall ....................................... 12
17 .T REND Backstrap Attachment to Concrete Parapet Wall ..............................1. 3 18 .C ompleted SENTRE System ..........................................................................1. 3
19 .C ompleted TREND System .........................................................................1. 4
20 .D amaged SENTRE Attenuator .....................................................................1.5..
21 .D amaged Southwest SENTRE Attenuator ...................................................1..7
22 .D ama.ge d TREND Attenuator ......................................................................1..8..
23 .D amaged TREND Attenuator .......................................................................1..8.
24 .O verla.pp. in.g Panels on Downstream Attenuator .............................................2. 0
25 . Overlapping Panels on Upstream Attenuator .................................................. 20
LlST OF TABLES
1 . Itemized List of Components Replaced (First SENTRE Hit) .............................. 16
2 .I temized List of Components Replaced (Second SENTRE Hit) ......................... 17 3 .I temized List of components ~eblaced(T REND Hit) .............: ... ......................1 9
4 . ATRC Recommended Stockpiled Parts ............................................................. 21
The AASHTO Guide For Selecting, Locating and Designing Traffic Barriers2
classifies the development of traffic barrier systems into three categories: 1)
Operational; a barrier system which has performed satisfactorily in crash tests and in
field evaluations. 2) Experimental; a barrier system which has performed satisfactorily
in crash tests, but for which there has yet to be sufficient in-service field evaluations. 3)
R & D; a barrier system for which there has not been sufficient crash tests or field
evaluations to draw a performance conclusion. When an agency receives federal
funding for the construction of a traffic barrier system classified as experimental by the
FHWA, that agency must agree to monitor and report on the in-service performance of
the barrier for a designated period of time. The performance data obtained is used in
conjunction with similar data provided by other projects nationwide by the FHWA to
determine if the barrier system will be upgraded to operational.
The SENTRE and TREND Systems
If an agency concentrates on the rigid bridge rail as the primary roadside
hazard, a solution to maintaining close quartered utility road access is a short
longitudinal barrier installed at the end of the bridge. One manufacturer, Energy
Absorption Systems, Inc. has developed two barrier systems which are short in length
and are claimed to meet NCHRP-230 crash test performance goals. These systems
are the TRansition END treatment (TREND) and the Safety barrier ENd TREatment
(SENTRE). The systems are similar, differing in that SENTRE is designed as a
guardrail end treatment and TREND is designed as a rigid barrier end treatment.
SENTRE has five telescoping thrie beam panels supported on vertical posts set on slip
bases. The slip bases are actually two plates welded together with the bottom plates
mounted on concrete footings or a concrete pad. TREND has six such thrie beam
panels and posts. Each system consists of a redirecting cable tightened to 100 foot-pounds
torque, and six sand boxes; four filled with 100 pounds of sand and two
containing 150 pounds of sand. The cable redirects vehicles from the roadside hazard,
and the sand boxes attenuate the impact. Manufacturer's drawings of SENTRE and
TREND are provided in Figures 5 and 6, respectively. A more detailed description of
the systems as provided by the manufacturer is included in Appendix C of this report.
Energy Absorption Systems, Inc. has presented certification reports supporting
claims that SENTRE and TREND meet the crash test performance requirements of
NCHRP-2303,4. The FHWA, concurring, but lacking sufficient in-service performance
data, had classified SENTRE and TREND as experimental barrier systems at the onset
of this project. In April, 1989, SENTRE was upgraded to an operational barrier system
by the FHWA. As of June, 1991 the TREND remains classified as experimental.
In-Service Performance Evaluation
The purpose of this project was to monitor and evaluate the in-sewice
performance of the SENTRE and TREND end treatments, based on NCHRP-230. In-service
evaluations are undergone with the objective of discovering what problems, if
any, may arise with the construction, operation, and maintenance of safety devices,
under a variety of circumstances at a typical site. These problems may not have been
evident during controlled crash tests. If such problems are discovered, modifications to
the devices may be proposed to improve the devices andlor lower costs prior to
widespread use.
The SENTRE and TREND systems were installed and monitored in accordance
with the FHWA approved workplan included in this report as Appendix A.
LOCATION OF PROJECTS
Originally, two sites were selected for the placement of TREND attenuating
systems. The sites were selected on the basis of design needs at both locations.
Research needs were not considered. It was later discovered that the steel bridge rail
plan for one of the selected locations precluded installation of the TREND, but that it
was suitable for a SENTRE installation. As a result, a second experimental project was
initiated for the SENTRE. The SENTRE installation is identified as Experimental
Project AZ-8802, and the TREND installation is identified as Experimental Project AZ-
8803.
AZ-8802: SENTRE End Treatment Field Installation
Four SENTRE end treatments were installed, one at each corner of a single
bridge, as part of ADOT construction project HES-02202(33)P. This bridge is located at
approximately milepost 138.0 on US 60, the Wickenburg-Phoenix Highway, and
crosses the Beardsley Canal. The posted speed limit at this location is 55 mph, and
the site is in ADOT District 3. The project involved removing existing concrete curb,
and replacing it with a new concrete cub supporting a tubular thrie beam bridge rail
assembly. The original 40' width of clear roadway, consisting of two 12' lanes and two
8' shoulders, was maintained. The highway is flat in the vicinity of the project and the
view is unobstructed. Figure 7 is a photo of the project site before construction.
The 1989 design ADT for this two-lane highway is 8,309 vehicles5. Sixty-two
percent of these are passenger type vehicles5. During the sixteen year period before
the onset of this project (1973-1988) there were 13 accidents involving collisions with
the existing bridge rail6. Six of those were injury accidents. The number of accidents
recorded divided by the number of years of the accident record yields a probability of
collision with the bridge rail of 0.813 for any one year. The corresponding probability of
at least one collision during the two year evaluation period is 0.965. These probabilities
are based solely on the number of previous incidents and no provision for changing
traffic conditions has been made.
The 1989 design ADT for this two-lane highway in the vicinity is 2,565 vehicles5.
Fifty-seven percent of these are passenger type vehicles5. During the sixteen year
period prior to the onset of this project (1973 to 1988) there were three reported
accidents at the canal bridge6. Based on this accident history the probability of an
accident at the canal bridge is 0.188 for any one year, and the corresponding
probability of at least one accident during the two year evaluation period is 0.340. The
probabilities presented here are based solely on the number of occurrences in the last
16 years and no provision for changing traffic conditions has been made.
CONSTRUCTION
Construction of the SENTRE system took place on December 19 and 21, 1988
and was performed by Klondyke Inc. The system was operational on December 21,
1988. The TREND system was installed on November 3, 1988 and was performed by
Nesbitt Contracting Co. The system was operational on November 3, 1988.
The construction procedures for the SENTRE and TREND were similar. The
forthcoming description applies to both systems unless otherwise stated. Plans for the
SENTRE and TREND projects are included as Appendix D and E of this report.
Procedure
A manufacturer's drawing of the assembly of post 2 is included as Figure 9 to
further aid in the description of the assembly of the SENTRE and TREND systems.
The SENTRE and TREND systems were constructed with concrete footings.
The concrete used was Class S concrete with fc= 4000 psi. The TREND footing is 21'
long, 4' wide, and 8" deep. The footing is increased to 3' deep in the 3' of the footing
furthermost from the bridge rail to accommodate the embedment of the redirecting
cable anchor. The SENTRE footing is the same except for its length. The SENTRE
footing is only 17.5' long as SENTRE only has five posts rather than TREND'S six.
The footings for the SENTRE system were poured on September 19, 1988 and
had a 28 day compressive strength of 4829 psi. The footings for the TREND system
were poured on October 17,1988 and had a 28 day compressive strength of 5839 psi.
The bottom plate of each of the slip bases is mounted on the concrete footings
with six 7.5" long, 314" diameter bolts. The SENTRE's five base plates were spaced at
36" on center and the TREND'S six base plates at 37.5" on center. The bolts were
epoxied into the holes with epoxy included as part of both the SENTRE and TREND
packages.
The upper plate of the slip base is mounted on the lower plate with four 2.5"
long 314" diameter bolts. This is illustrated in Figure 10.
The support posts of the SENTRE and TREND consist of 32 long W6.5~9 A36
steel posts with a slotted 112" thick steel plate welded to the end. The support posts
are bolted to the slip bases at these plates as shown in Figure 11. The bolts are
tightened to manufacturers specification of 60 ft-lbs torque.
Figure 11 Post Attached To Base Plate.
A 21" W6.5~9 steel blockout is attached to each post. The systems' thrie beam
panels are fixed to these blockouts. The panels are connected together by a
mushroom bolt in horizontal slots and overlap such that the overhang is away from the
free end of the system.
The three posts furthest from the bridge (posts labeled 1, 2, and 3 in figures 5
and 6) have sand containers attached. Posts 1 and 2 each support two containers,
both designed to hold 100 lbs. of sand. Post 3 supports two containers, each with 150
Ibs. of sand. The containers were filled with sand and the lids snapped shut.
A 23' steel redirecting cable goes through a hole in post 1 and is anchored at
the front of the system. The other end is fastened at a rear anchor location forming an
angle of approximately 25 degrees with the roadway. The cable is tightened to a
specified 100 ft-lbs torque. The cable and anchors are shown in Figures 12 and 13.
A 20' transition between the SENTRE and the tubular thrie beam bridge rail was
placed upon seven posts driven into the ground spaced at 3' O.C.. The posts are the
same type of post as used in the SENTRE system. The end panel of the SENTRE is
tied to the transition. As the transition is also of thrie beam configuration, it is important
to insure the concave component of the SENTRE and transition thrie beams are
concurrent. This special requirement was not foreseen in the construction of this
installation. The upstream thrie beam matched the SENTRE panels, but the
downstream splice was of opposite concavity to the SENTRE panels. The problem
required a special downstream splice. The special downstream splice and an upstream
splice are shown in Figures 14 and 15.
Figure 12 Front Anchor of Redirecting Cable.
Figure 13 Rear Anchor of ~edirectionC able.
.. -
Figure 14 Special Downstream Splice.
1 I
. An itemized list of the replaced components and associated costs is shown in
Table 1. The unit prices are those provided by EASl at the time of replacement.
DESCRIPTION UNIT PRICE QUANTITY COST
Post 1 $73.00 1 $73.00
Post 2 65.00 1 65.00
Post 3 50.00 1 50.00
Blockout 16.00 2 32.00
Nose 50.00 1 50.00
l00# Sand Box 35.00 2 70.00
Tax and Shipping 50.92
Labor 215.08
Total $606.00
Table 1 Itemized List of Replacement Costs (First SENTRE Hit).
The thrie beam fender panel was also damaged by this first impact, suffering a
slight crimp. Because of the minor amount of damaged sustained, and the substantial
cost of replacement ($227), it was decided that the crimp would be pounded out, as
opposed to replacing the panel.
Energy Absorption Systems, Inc. had claimed that only 48 hours were required
to receive replacement parts from the time of ordering. This was not the case. The
replacement parts of Table 1 were ordered on August 31, 1989 and not received by
ADOT until September 14, 1989. As a result, the attenuator was out of sewice for over.
three weeks.
According to the work report filed by District 3 Maintenance Division, the actual
repair process was simple and was carried out by three men working a total of eight
hours each. The total cost of the repair including parts, labor, shipping and tax came to
$606.
The second impact was discovered September 26, 1989. The Maricopa county
road construction project was still in progress on US 60, and the highway was still
temporarily realigned beginning just west of the SENTRE attenuators. As with the first
incident, there was a significant amount of heavy construction equipment and semi-truck
traffic. Two of the attenuators were discovered to be damaged; the southwest
more severely than the northwest. At the southwest attenuator, marks in the soil and
impact induced spalling of the SENTRE's concrete footing sewed as evidence that a
front end loader or scraper had scraped its bucket or blade all the way up to the
attenuator. The southwest attenuator was obviously impacted by a large construction
vehicle, possibly the same vehicle that had left the scrape marks in the soil. This
attenuator was impacted in the direction of travel and telescoping of the panels did
occur. The mushroom bolt attached to panel 1 was free to slide as designed. A
photograph of the damaged system is included as Figure 21.
Figure 21 Damaged Southwest SENTRE Attenuator.
The northwest attenuator was hit on the nose longitudinally. Post 1 was slightly
twisted but judged operational. An itemized list of the items replaced and associated
1989 costs is given in Table 2.
DESCRIPTION UNIT PRICE QUANTITY COST
Post 1 (SW) $73.00 1 $73.00
Post 2 (SW) 65.00 1 65.00
Blockout (SW) 16.00 2 32.00
Nose (SW) 50.00 1 50.00
Nose (NW) 50.00 1 50.00
100# Sand Cont. (SW) 35.00 2 70.00
Tax and Shipping 23.00
Labor ' 236.80
Total $599.80
Table 2 Itemized List of Replacement Costs (Second SENTRE Hit).
Once again EASl's claim of requiring only 48 hours from order to delivery of
replacement parts proved not to be the case. As a result the attenuator was out of
service for nearly a month.
The damaged TREND system was no longer parallel with the roadway, and it's
future performance was doubtful. As such, Energy Absorption Systems, Inc. was
contacted for replacement parts and the system was repaired. Table 3 shows an
itemized list of components replaced and associated costs from EASl at the time of
replacement.
DESCRIPTION UNIT PRICE QUANTITY COST
Post 1 $73.00 1 $73.00
Post 2 65.00 2 130.00
314" Bolt, 2.5" 0.42 4 1.68
314" Washer 0.25 4 1 .OO
314" Nut 0.47 4 1.88
Blockout 16.00 2 32.00
Nose 50.00 1 50.00
100# Sand Cont. 35.00 2 70.00
Tension Strap, 79" 1 1 .OO 1 11 .OO
Tax and Shipping 56.90
Labor 1 12.97
Total $540.41
Table 3 Itemized List of Components Replaced (TREND Hit).
The thrie beam fender panel was also slightly dented. The minor amount of
damage sustained combined with the substantial replacement cost ($227) resulted in a
decision not to replace the component.
As with the SENTRE system, EASl could not maintain their claim of only 48
hours order-to-delivery time. Parts ordered on February 9 were not received by ADOT
until March 3. As a result, the attenuator was out of service for 37 days.
The repair was performed by an ADOT maintenance crew. The crew
dismantled and removed the damaged components, and replaced them with the new
parts. The crew of three men worked 4 hours, for a total of 12 man-hours, to complete
the job. The labor cost was $1 13, bringing the total cost or repair including parts, labor,
tax, and shipping to $540.
The extent of the damage to the TREND caused by vandals was the
unfastening and removal of the plastic lids of the sand containers. This happened on
several occasions and was corrected by merely re-fastening the lid by the inspecting
maintenance personnel.
In-Service Performance
During the course of the evaluation period three of the SENTRE and one of the
TREND attenuators had been damaged severely enough to require repair. All of these
impacts appeared to have been the result of heavy construction equipment. Because
in each of the incidents the vehicle did not remain at the site, it is inappropriate to draw
any conclusions on the safety performance of the attenuators per NCHRP-230. Due to
the lack of any reports of the collisions, it is reasonable to assume that in all instances
the occupants of the vehicle were not injured.
Although the impacts did not lend themselves to a safety performance
evaluation of the SENTRE and TREND, in-service performance information of the
systems was provided. After each of the impacts, the spare parts required for repair
were ordered from Energy Absorption Systems, Inc. promptly. In every instance it took
within a range of two weeks to 37 days for the replacement parts to be received by
ADOT. EASl had claimed a maximum of 48 hours order-to-delivery time for
replacement parts.
In each case, the repair of the SENTRE and TREND systems was simple, and
completed within a day by a three man maintenance crew.
Future Conslderations
As a result of the repeated excessive delay in delivery of replacement
components for the SENTRE and TREND, the Arizona Transportation Research Center
(ATRC), working with EASl representatives, had recommended that ADOT stockpile
one set of specific spare attenuator parts at a cost of approximately $1079. An
itemized list of the parts recommended by the ATRC is shown in Table 4.
DESCRIPTION UNIT PRICE QUANTITY COST
Post 1 $73.00 2 $146.00
Post 2 65.00 2 130.00
Post 3 50.00 3 150.00
Blockout 16.00 6 96.00
Fender Panel 227.00 1 227.00
100# Sand Cont. 35.00 4 140.00
150# Sand Cont. 45.00 2 90.00
Nose 50.00 2 100.00
Total $1079.00
Table 4 ATRC Recommended Stockpiled Parts.
In May of 1990, ADOT District 3 Maintenance purchased the spare parts of
Table 4 for maintenance of the SENTRE system. The total cost after tax and delivery
came to $1 174. There have been no incidents requiring repair of the attenuator system
since the acquisition of the spare parts. ADOT District 2 thusfar has not purchased any
spare parts in advance for the TREND system.
A concern brought to light by this experimental project, and subsequent
stockpiling of parts, is the problem with the proliferation of numerous end treatments. A
number of different end treatments with non-interchangeable components will require
substantially more warehouse space than only one or two acceptable end treatments.
RECOMMENDATIONS
Based on the experiences of the field installations of SENTRE and TREND in
Arizona, the following recommendations are presented:
1. Agencies should stockpile replacement parts for highway
appurtenances that may require repair. The timely repair or replacement
of attenuators and other appurtenances is crucial from both a safety and
liability point of view.
2. Agencies should utilize similar appurtenances in geographical
proximity to one another. The reason for this is to expedite repair by
allowing stockpiled parts to be kept nearby, rather than at a central
location. If two or more types of appurtenances without interchangeable
parts are close together, the storage space and capital investment
needed for stockpiled parts will be significantly increased.
3. Due to the nature of the impacts encountered, the TREND should be
subject to further monitoring until such a time that the FHWA has
developed sufficient database to determine if the system should be
upgraded to operational. The SENTRE was upgraded to operational in
April, 1989.
REFERENCES
1. Michie, Jarvis D., "Recommended Procedures for the Safety Performance
Evaluation of Highway Appurtenances." NCHRP Report 230 (March 1981).
2. American Association of State Highway and Transportation Officials, "Guide for
Selecting, Locating, and Designing Traffic Barriers." (1977).
3. Energy Absorption Systems, Inc., "SENTRE (Safety Barrier End Treatment) NCHRP
230 Certification Report." (May 1983).
4. Energy Absorption Systems, Inc., "TREND (Transition End Treatment) NCHRP 230
Certification Report." (December 1985).
5. 1989 Traffic Design Data, ADOT Materials Pavement Services.
6. Accident Location Identification and Surveillance System (ALISS), ADOT Traffic
Studies Branch.
7. Construction Costs 1987, ADOT Contracts and Specifications.
8. Construction Costs 1988, ADOT Contracts and Specifications.
9. Lattin, Douglas J., "SENTRE and TREND Attenuator Field Installations,
Construction Report", Arizona Department of Transportation (February, 1990).
APPENDIX A - FHWA APPROVED WORKPLAN
AZ-8802 (SENTRE)
AZ-8803 (TREND)
WORKPIAN
1. Evaluate and document the site selection criteria, and design conditions. This will include
expected service requirements of the project and anticipated service life of the device.
The geometric alignment, device location, traffic volume, vehicle operating speeds and
mix, environmental conditions, and soil stratigraphy will be documented.
2. Perform a risk analysis and pre-installation safety evaluation. Traffic and accident data will
be obtained and analyzed for each instaliation for the period 2 years prior to award of the
construction project. An appropriate risk analysis program will be selected and used to
determine the probability for collision for each device. Local maintenance authorities/ DPS
officers will be interviewed to determine if any unique safety or environmental conditions
are prevalent.
3. Assign reporting procedures and responsibilities. The ATRC will develop field evaluation
forms for use by ADOT construction and maintenance personnel and DPS officers. The
frequency and content of reporting will be established by the ATRC in conjunction with the
participating personnel.
4. Monitor and document the construction of the devices. The as-built condition of each
device and roadway condition wiil be documented and an emphasis placed on verifying
that design goals were achieved. Roadway friction testing will be conducted to document
skid properties at the time of construction. Field construction/contractor personnel will be
interviewed for suggested design/ procedural changes and/or improvements.
5. Prepare a construction report documenting the design and construction of the devices.
A construction report will be prepared in accordance with ATRC procedures for reporting
of experimental projects and submitted to the FHWA within 120 days after construction of
the last device.
6. Monitor in-service performance.
* The in-service evaluation will be conducted for 2 years,
Monthly field inspections will be performed by ADOT maintenance forces to
record 'brush hits" and drive away collisions, damage to the appurtenance,
required repairs, routine maintenance, and evidence of near misses. The
availability of replacement parts, level of technical support by the supplier, and
total down lime of the appurtenance will be documented. Unique problems such
as vandalism or corrosion will be identified.
* Reported accidents will be investigated by the ATRC as required. Damage to
appurtenances will be documented and video taped. Accident reporting will be
performed using techniques of the National Accident Sampling System or other
acceptable procedures.
* Traffic volume and mix will be obtained annually.
* ATRC wiil perform scheduled inspections of the installation at 6 months. 1 year,
and 2 years after construction. Interviews with maintenance personnel and DPS
officers will be pe&ormed annually.
* Annual maintenance costs will be collected by the ATRC.
7. Evaluate in-sewice performance. A before and after evaluation will be performed which
evaluates the relative effectiveness of the appurtenances. Specific appurtenance
performance will be evaluated on the basis of three factors: structural adequacy, occupant
risk, and vehicle trajectory after collision. The evaluation criteria are as follows:
a) Structural Adequacy: Measure of geometrical, structural and dynamic properties
of an appurtenance to interact with a selected range of vehicle sizes and impact
conditions in a predictable and acceptable manner. Nonvehicie colllslon-type
forces such as wind are not included. Criteria:
Acceptable redirection of vehicle.
Controlled penetration of vehicle.
Controlled stopping of a selected range of vehicle sizes impacting the installations
at spectied conditions.
Detached elements, fragments, or other debris should not penetrate or show
potential for penetrating the passenger compartment, or present undue hazard to
other traffic.
b) Occupant Risk: Vehicle responses of acceleration and velocity changes. Criteria:
Vehicle remains upright during and after collision although moderate roll, pitching,
and yawing are acceptable.
Minimize velocity change in vehicle. Small cars at both low and high impact
speeds are the critical test.
Minimize vehicle velocity change prlor to occupant impact
c) Vehicle Trajectory: Criteria:
Vehicle trajectory and final stopping position should intrude a minimum distance, ff
at all, into adjacent or opposing traffic.
For longitudinal barrier terminals, vehicle trajectory behind the test article is
acceptable in theory.
The evaluation will determine if the design goals were achieved, identify special problems
that affected appurtenance performance, examine impact the devices exhibited on other
highway conditions, and document the initial cost and annualized maintenance cost.
8. Prepare a final report A final report detailing the efforts of this study and the conclusions
and recommendations will be prepared. This report will be prepared within 90 days of the
completion of the final evaluation in accordance with ATRC procedures for reporting
experimental projects.
APPENDIX B - CURRENT ADOT STANDARD DETAILS
APPENDIX C - EASl TECHNICAL DISCUSSION
TECHNICAL DISCUSSION
TEST CONDITIONS
Test F a c i l i t y
The Energy Absorption Sy s~~.nIsn c. t e s t f a c i l i t y i s located a t the Lincoln
Airport i n Lincoln, California. The test area i s situated on f u l l y asphalted
level ground and has been cleared of a l l obstructions f o r unrestricted t r a j e c t o r y
of the vehicle. The s o i l i s composed of very s t i f f to hard s i l t s and clays and
can be c l a s s i f i e d as a f4CHRP 230 type S-1.
Test A r t i c l e (Design)
The SENTRE (see Figure 0-1) has been designed and constructed to provide
structural adequacy minimum occupant r i s k and minimum vehicle t r a j e c t o r y as Set
7 )
f o r t h i n NCHRP 230 Table 3--. "Crash Test Conditions f o r Minimum Matrix".
(Table 1)
The SENTRE i s designed as an end treatment f o r w-beam or t h r i e beam guard-r
a i l which w i l l redirect the nose of the impacting vehicle away from the
unyielding guardrail while a t the same time dissipate the energy of the impacting
vehicle.
The SENTRE consists of f i v e nested overlapping t h r i e beam fender panels
which telescope rearward i n response to a longitudinal impact force and an angled
side cable f o r urging the f i r s t fender panel and post assembly l a t e r a l l y away
from the fixed guardrail end. The fender panels and angle side cable function to
direct the nose of the impacting vehicle away from the hard point on the
guardrail while at the same time dissipating the impact energy of the vehicle.
The fender panels are slotted and secured together i n a nested fashion by
fasteners which allow the fender panels to telescope upon the application of an
axial impact force. The fender panels are supported above the ground on v e r t i c a l
support posts which are positioned on s l i p bases. These s l i p bases allow the
port< t o break away from submerged ground anchors so t h a t the fender panels may
telescope.
The f i r s t fender panel, or more s p e c i f i c a l l y i t s v e r t i c a l support post i s
connected to a r e d i r e c t i o n i n g cable. This cable i s secured t o an anchor located
a t the f r o n t of the u n i t , and a rear anchor located a t a l a t e r a l p o s i t i o n away
" f r o m t h e g u a r d r a i l . This cable i s positioned so t h a t when a l o n g i t u d i n a l impact
force i s imposed on the f r o n t o f the system, the cable w i l l urge the f i r s t fender
panel l a t e r a l l y as i t telescopes rearward. The l a t e r a l force o f the cable and
f i r s t Post i n conjunction with l a t e r a l forces contributed by the subsequent posts
w i l l urge the vehicle away from the hard p o i n t on the g u a r d r a i l .
Test A r t i c l e (Construction)
The SENTRE drawings are shown i n Appendix 0. The f o l l o w i n g discussion w i l l
describe how the i n d i v i d u a l components are constructed and assembled i n t o a
working u n i t .
The 52 inch, I0 gauge t h r i e beam fender panels include 32 inch s l o t s and are
secured together by fasteners (mushroom b o l t assembly). These s l o t s allow the
fender panels t o telescope upon the a p p l i c a t i o n o f a l o n g i t u d i n a l force.
The mushroom b o l t assembly i s designed with a shoulder t h a t t r a v e l s i n the
s l o t o f the fender panel. The assembly secures two overlapping fender panels w i t h
a grade 5, f l a t head 3" x 5/8" diameter b o l t which passes through a hole i n the
center o f the mushroom washer and a hole i n the underlying fender panel. The
mushroom b o l t assembly i s constructed so t h a t it does not s o l i d l y clamp the two
fender panels together, but rather secures them i n a p o s i t i o n r e l a t i v e t o one
another with s u f f i c i e n t tolerance t o allow the f i r s t fender panel t o telescope
i n t o the second panel. The l o n g i t u d i n a l movement o f the f i r s t fender panel i s
halted when it reaches the end o f the s l o t .
The fender panels are supported above the ground by v e r t i c a l support posts.
The 32" l o n g pos t s a r e W 6.5 x 9 s t e e l "I"be ams t o which an a d d i t i o n a l 21 i n c h W
6.5 x 9 "I" beam blockout i s bolted with two 1 1/2" x 3/4" diame'er b o l t s . The
fender panels are then attached t o the blockout with two 2" x 3/J" diameter grade
2 bolts. The purpose o f the blockout i s t o prevent automobiles w i t h small wheels
from snagging on the v e r t i c a l support posts o f the SENTRE during a side angle
impact.
The v e r t i c a l support posts are welded t o a 1/2" x 8" x l i ' steel s l i p base.
The s l i p base assembly includes a top p l a t e and a bottom p l a t e which are secured
t o each other. The bottom p l a t e i s attached t o an earth anchor.
The top and bottom s l i p base plates each include four open ended s l o t s which
are designed to receive 2" x 3/4" diameter b o l t s which secure the plates
together. The plates are large enough so t h a t they w i l ' l not y i e l d during a
l a t e r a l impact. The s l o t s are open ended so t h a t when a s u f f i c i e n t l o n g i t u d i n a l
impact force i s applied t o the v e r t i c a l support post by the impacting vehicle,
the plates w i l l s l i d e apart. To insure t h a t the plates w i l l s l i p apart i n a
predictable manner, they are separated by four 3/4" diameter f l a t washers. The
washers provide a consistant bearing area between the two plates so t h a t the
force needed t o cause the plates to s l i d e can be c o n t r o l l e d . Testing has shown
t h a t t h e vehicle sustains acceleration l e v e l s o f 4 t o 5 "Gs" when a torque of 60
f t . - l b s . i s applied t o the four s l i p base bolts. 22
The v e r t i c a l suoport posts also include a 4" x 4" steel gusset attached from
the v e r t i c a l support post to the top of the s l i p base p l a t e . This gusset
strengthens the v e r t i c a l support p o s t d u r i n g r e d i r e c t i v e impacts.
An additional 3'' x 6" angle p l a t e i s welded to the bottom s l i p base t o
provide a ramp and prevent possible snagging on each other as they break away and
move rearward i n response t o a l o n g i t u d i n a l impact force.
The f i r s t v e r t i c a l support post i s s i m i l a r i n construction t o the other
posts except t h a t it contains a 1 3/4" x 2" diameter schedule 80 steel pipe
gromet. The gromnet i s located 1 1/2" from t h e t o p o f the s l i p base and i s
designed to receive a 1 1/2" diameter threaded steer f i t t i n g which i s swedged to
the end of a 7/8" diameter steel cable. The cable extends from the previously
. .
mentioned front cable anchor, through the gromnet, to the rear cable anchor.
The rear anchor is located on an imaginary line which runs through the center of
the f i r s t vertical post a t an angle of 25 degrees w i t h respect to the centerline
of the roadway. The cable forces the f i r s t fender panel and vertical post to
move l a t e r a l l y upon the application of a longitudinal impact force.
The front and rear cable anchors are typically embedded in a concrete
foundation measuring 18" diameter by 4 feet deep. The front and rear anchor
consist of a 1" x 3" x 29'' steel bar welded to a 1/2" x 5" x 7" plate. The
anchors are designed to be universal and secure each end of the cable. The front
cable anchor i s positioned ahead of the f i r s t vertical support post and secures
the clevis end of the cable using 1 5/8 " diameter pin and cotter pin. The cable
passes through the gromnet in the f i r s t vertical support post and i s then secured
to the rear cable anchor by inserting the threaded f i t t i n g on the end of the
cable through the 1 3/4" diameter hole in the steel anchor and attaching a washer
and nut. The I 1/2" nut i s torqued to approximately 100 f t . - l b s . The cable
aids in redirectioning vehicles which impact the SENTRE head on. By urging the
f i r s t fender panel l a t e r a l l y the cable imposes a lateral force on the fender
panels. The cable i s constructed from 7/8". 6 x 25 IWRC. galvanized. steel cable
and will stretch 1 to 1 1/2 % of i t s length upon application of a longitudinal
impact force.
The lateral force will now be described in more detail. When a vehicle
impacts the guardrail end terminal head on. the f i r s t panel i s forced backwards
telescoping into the second panel. As the vehicle continues i t s motion, the
f i r s t vertical post impacts a second vertical support post causing the top plate
of the second s l i p base to disengage. The rearward movement of the f i r s t panel
stretches the cable until the cable will not stretch any further. The cable then
urges the f i r s t panel l a t e r a l l y causing the f i r s t fender panel t o give a smal 1
l a t e r a l impulse t o the nose o f the impacting vehicle. As the f i r s t fender panel
. .
reaches the end o f i t s t r a v e l the second fender panel begins t o telescope i n t o
the t h i r d fender panel. The f i r s t fender panel reaches the end Of i t s
- , l o n g i t u d i n a l movement before the second s l i p base breaks free. Each s l i p base
.decelerates and dissipates some of the energy of the impacting vehicle. This
process continues u n t i l a l l the s l i p bases of the SENTRE have disengaged g i v i n g a
large l a t e r a l force t o the impacting vehicle. The net consequence o f t h i s l a t e r a l
force moves'the vehicle away from the hard point.
The SENTRE includes additional mass i n the form o f sand because the s l i p
bases do not remove a s u f f i c i e n t amount o f energy t o keep an impacting vehicle
from h i t t i n g the hard point. The sand i s held i n containers which add 200 l b s .
t o the f i r s t and second v e r t f c a l support posts a t a 24 i n . center o f g r a v i t y and
300 lbs. t o the t h i r d v e r t i c a l support post a t a 21 in. center o f g r a v i t y . The
200 lbs. o f sand i s e q u a l l y d i v i d e d i n t o two 100 l b . containers f o r each of the
f i r s t two posts. A 100 lb. container i s placed on each side o f the block out and
v e r t i c a l post so t h a t two 2" x 3/8" diameter b o l t s can be inserted and clamp the
containers t o the v e r t i c a l post assembly. The 300 l b . sand mass i s equally
divided i n t o two 150 lb. sand contaners and attached t o the t h i r d post i n the
same manner as the 100 l b . sand containers. A l i d i s included on each container
t o keep moisture from entering the sand. The l i d i s designed w i t h a s e l f l o c k i n g
feature so no assembly tools such as r i v e t s are required.
A b u l l nose has been included as p a r t o f the design t o aid i n the aesthetic
appeal. The b u l l nose i s made o f gray p l a s t i c and includes a f l a t section on the
nose which may be used t o attach r e f l e c t i v e markers.
The SENTRE may be attached t o e i t h e r w-beam or t h r i e beam g u a r d r a i l . Both
guardrail types must include an end anchoring system which extends from a ground
anchor t o the section o f guardrail imnediately f o l l o w i n g the SENTRE. A
t r a n s i t i o n panel must be included when i n s t a l l i n g the SENTRE on w-beam g u a r d r a i l .
The t r a n s i t i o n panel i s connected from the l a s t fender panel o f the SENTRE t o :he
. .
hard p o i n t of the length o f need g u a r d r a i l . A 7/8" 6 x 19 IWRC cable extends from
a concrete deadman, located betweeen posts 4 and 5, t o a cable anchor on. the
t r a n s i t i o n panel. The cable and concrete deadman are strong enough so t h a t the
L.O.N. guardrail deveIops i t s f u l l t e n s i l e strength d u r i n g a r e d f r e c t impact.
APPENDIX D - CONSTRUCTION PLANS FOR AZ-8802 (SENTRE)
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APPENDIX E -CONSTRUCTION PLANS FOR AZ-8803 (TREND)

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Copyright to this resource is held by the creating agency and is provided here for educational purposes only. It may not be downloaded, reproduced or distributed in any format without written permission of the creating agency. Any attempt to circumvent the access controls placed on this file is a violation of United States and international copyright laws, and is subject to criminal prosecution.

ARIZONA DEPARTMENT OF TRANSPORTATION
REPORT NUMBER: FHWA-AZ-8802 & FHWA-AZ-8803
SENTRE AND TREND
ATTENUATING SYSTEMS
Final Report
Prepared by:
Greg Rollins
Lany A. Smfield
Arizona Transportation Research Center
College of Engineering & Applied Sciences
Arizom State University
Tempe, Arizona 85287
July I991
Prepared for:
Arizona Department of Transportation
206 South 17th Avenue
Phoenk, Arkona 85007
in rooperation with
U.S. Depaltment of Transportation
Federal Highway Administration
The contents of this report reflect the views of the authors who are responsible for the facts and the
accuracy of the data presented herein. The contents do not necessarily reflect the official views or
policies of the Ariiona Department of Transportation or the Federal Highway Administration. This
report does not constaute a standard, specification, or regulation Trade or manufacture* names
which may appear herein are cited only because they are considered essential to the objectives of
the report. The U. S. Gwernment and the State of Arizona do not endorse products or
manufacturers.
Technical Report Documentation Pagi
I. Report No. 2. Government Accession No. 3. Rscipientb Catalog No.
MWA-AZ-8802 & FHWA-AZ-8803
I
1. Tide and Subtitle 5. Report Date
Julv 1991
SENTRE and TREND Attenuating Systems 6. Performing Organization Code
I
7. Author (s) 8. Performing OrganIration Report No.
Greg Rollins and Lany A. Scofield
3. Performing Organizdon Name and Address 10. Work Unit No.
ARIZONA TRANSPORTATION RESEARCH CENTER
206 S. l7TH AVENUE, MAIL DROP 075R 11. Contract or &ant No.
PHOENIX, ARIZONA 85007 HPR-PL-l(37) ITEM 114
12. Sponsoring Agency Name and Address 13.Type of Report & Period Covered
ARIZONA DEPARTMENT OF TRANSPORTATION FINAL Dec. 1988 -Jan. 1991
206 S. 17TH AVENUE
PHOENIX, ARIZONA 85007 14. Sponsoring Agency Code
I
15. Supplementary Notes
Prepared in cooperation with the U.S. Department of Transportation, Federal Highway Administration
The objective of this study was to construct SENTRE and TREND attenuator systems and evaluate their
in-sewice performance as set forth in NCHRP-230 for a period of two years. The evaluation was conducted
as part of ADOTs New Product Evaluation Program. ~ o tohf the systems had performed satisfactorily in full
scale crash testing conducted during other research efforts, and were designated as experimental by ihe
FHWA at the onset of the study.
Four SENTRE end treatments were installed, one downstream and one upstream on both sides of a
single bridge, as part of ADOT construction project HES-02202(33)P. Likewise, four TREND end treatments
were installed, one downstream and one upstream on both sides of a single bridge, as part of ADOT
construction project F-081-l(2). Both of these bridges extend over canals. The canals have parallel utility
roads, and access to these roads precludes the full development of the length-of-need (LON) required with
the use of guardrail. Following construction, the in-sewice performance was assessed from accident and
maintenance cost records.
The systems were constructed without major difficulty, the TREND was completed and operational
November 3,1988, and the SENTRE was operational December 22,1988. During the evaluation period the
SENTRE was hi twice and the TREND was hi once. None of the impacts lent themselves to a safety
performance evaluation of the attenuators, however, each of these hits required repair and replacement parts
for the attenuators.
The repair process was found to be simple and in each case was performed without difficulty. However,
the delivery time for replacement parts far exceeded EASl's claim of 48 hours. Delivery times ranged from
two weeks to 37 days. The ATRC has recommended stockpiling spare attenuator parts in proximity to the
attenuators for their timely repair. Further monitoring of the TREND is recommended until it is judged
operational by the FHWA.
Wenuator, End Treatment, Transition, End
rerminal, Attenuating System, SENTRE, TREND,
Zuardrail, Bridge Rail, Traffic Barrier,
4ppurtenance.
Unclassified I Unclassified I
18. Distribution Statement
Document is available to the U.S.
Public through the National
Technical Information Sewice,
Springfield, Virginia, 22161
TABLE OF CONTENTS
INTRODUCTION ...................................................................................................... 1
Problem Statement ..............................................................................................1..
ADOT Standard End Treatments. .....................................................................2.. ...
Traffic Barrier Performance Criteria ....................................................................3.. .
The SENTRE and TREND Systems .....................................................................4.
OBJECTIVES .........................................................................................................6..
In-Service Performance Evaluation ......................................................................6.
LOCATION OF PROJECTS ..................................................................................... 6
AZ-8802: SENTRE End Treatment Field Installation ............................................ 6
AZ-8803: TREND End Treatment Field Installation .............................................7.
CONSTRUCTION ...................................................................................................8..
Procedure ...........................................................................................................8.. .
ECONOMICS .......................................................................................................... 14
Construction Cost Comparison ............................................................................ 14
Maintenance Costs .............................................................................................. 14
INCIDENTS ..........................................................................................................1..5
The SENTRE System .......................................................................................... 15
The TREND System ............................................................................................1 8
CONCLUSIONS ..................................................................................................2..0.
Construction ......................................................................................................2..0
In-Service Performance ....................................................................................... 21
Future Considerations .......................................................................................2..1
RECOMMENDATIONS. ........................................................................................2..2
REFERENCES .....................................................................................................2..3
APPENDIX A - FHWA Approved Workplan .........................................................A..1.
APPENDIX B Current ADOT Standard Details ...................................................B..1.
APPENDIX C - EASl Technical Discussion ........................................................C....l
APPENDIX D - Construction Plans for AZ-8802 (SENTRE) ..................................D. l
APPENDIX E - Construction Plans for AZ-8803 (TREND) .....................................E. l
LlST OF FIGURES
. .
1 .C anal and Utlllty Road ......................................................................................1. ..
2 .B ridge With Full LON .......................................................................................2..
3 .A Breakaway Cable Terminal (BCT) ....................................................................2
4 . An ADOT Standard Attenuator Assembly ..................................................... 3 5 .S ENTRE End Treatment ...................................................................................5..
6 .T REND End Treatment ......................................................................................5..
7 . AZ-8802: SENTRE End Treatment Installation Site Prior to Construction .......... 7
8 .A Z-8803: TREND End Treatment Installation Site Prior to Construction ............7
9 . Assembly of SENTRE and TREND Post 2 ...................................................... 9
10 .B ase Plate Assembly .......................................................................................9..
1 1 .P ost Attached To Base Plate. ..........................................................................1 0
12 .F ront Anchor of Redirecting Cable ................................................................1..1
13 .R ear Anchor of Redirecting Cable .................................................................1..1
14 .S pecial Downstream Splice ............................................................................. 1 1
15 . Proper Splice ................................................................................................... 12
16 . TREND Attached to Front of Concrete Parapet Wall ....................................... 12
17 .T REND Backstrap Attachment to Concrete Parapet Wall ..............................1. 3 18 .C ompleted SENTRE System ..........................................................................1. 3
19 .C ompleted TREND System .........................................................................1. 4
20 .D amaged SENTRE Attenuator .....................................................................1.5..
21 .D amaged Southwest SENTRE Attenuator ...................................................1..7
22 .D ama.ge d TREND Attenuator ......................................................................1..8..
23 .D amaged TREND Attenuator .......................................................................1..8.
24 .O verla.pp. in.g Panels on Downstream Attenuator .............................................2. 0
25 . Overlapping Panels on Upstream Attenuator .................................................. 20
LlST OF TABLES
1 . Itemized List of Components Replaced (First SENTRE Hit) .............................. 16
2 .I temized List of Components Replaced (Second SENTRE Hit) ......................... 17 3 .I temized List of components ~eblaced(T REND Hit) .............: ... ......................1 9
4 . ATRC Recommended Stockpiled Parts ............................................................. 21
The AASHTO Guide For Selecting, Locating and Designing Traffic Barriers2
classifies the development of traffic barrier systems into three categories: 1)
Operational; a barrier system which has performed satisfactorily in crash tests and in
field evaluations. 2) Experimental; a barrier system which has performed satisfactorily
in crash tests, but for which there has yet to be sufficient in-service field evaluations. 3)
R & D; a barrier system for which there has not been sufficient crash tests or field
evaluations to draw a performance conclusion. When an agency receives federal
funding for the construction of a traffic barrier system classified as experimental by the
FHWA, that agency must agree to monitor and report on the in-service performance of
the barrier for a designated period of time. The performance data obtained is used in
conjunction with similar data provided by other projects nationwide by the FHWA to
determine if the barrier system will be upgraded to operational.
The SENTRE and TREND Systems
If an agency concentrates on the rigid bridge rail as the primary roadside
hazard, a solution to maintaining close quartered utility road access is a short
longitudinal barrier installed at the end of the bridge. One manufacturer, Energy
Absorption Systems, Inc. has developed two barrier systems which are short in length
and are claimed to meet NCHRP-230 crash test performance goals. These systems
are the TRansition END treatment (TREND) and the Safety barrier ENd TREatment
(SENTRE). The systems are similar, differing in that SENTRE is designed as a
guardrail end treatment and TREND is designed as a rigid barrier end treatment.
SENTRE has five telescoping thrie beam panels supported on vertical posts set on slip
bases. The slip bases are actually two plates welded together with the bottom plates
mounted on concrete footings or a concrete pad. TREND has six such thrie beam
panels and posts. Each system consists of a redirecting cable tightened to 100 foot-pounds
torque, and six sand boxes; four filled with 100 pounds of sand and two
containing 150 pounds of sand. The cable redirects vehicles from the roadside hazard,
and the sand boxes attenuate the impact. Manufacturer's drawings of SENTRE and
TREND are provided in Figures 5 and 6, respectively. A more detailed description of
the systems as provided by the manufacturer is included in Appendix C of this report.
Energy Absorption Systems, Inc. has presented certification reports supporting
claims that SENTRE and TREND meet the crash test performance requirements of
NCHRP-2303,4. The FHWA, concurring, but lacking sufficient in-service performance
data, had classified SENTRE and TREND as experimental barrier systems at the onset
of this project. In April, 1989, SENTRE was upgraded to an operational barrier system
by the FHWA. As of June, 1991 the TREND remains classified as experimental.
In-Service Performance Evaluation
The purpose of this project was to monitor and evaluate the in-sewice
performance of the SENTRE and TREND end treatments, based on NCHRP-230. In-service
evaluations are undergone with the objective of discovering what problems, if
any, may arise with the construction, operation, and maintenance of safety devices,
under a variety of circumstances at a typical site. These problems may not have been
evident during controlled crash tests. If such problems are discovered, modifications to
the devices may be proposed to improve the devices andlor lower costs prior to
widespread use.
The SENTRE and TREND systems were installed and monitored in accordance
with the FHWA approved workplan included in this report as Appendix A.
LOCATION OF PROJECTS
Originally, two sites were selected for the placement of TREND attenuating
systems. The sites were selected on the basis of design needs at both locations.
Research needs were not considered. It was later discovered that the steel bridge rail
plan for one of the selected locations precluded installation of the TREND, but that it
was suitable for a SENTRE installation. As a result, a second experimental project was
initiated for the SENTRE. The SENTRE installation is identified as Experimental
Project AZ-8802, and the TREND installation is identified as Experimental Project AZ-
8803.
AZ-8802: SENTRE End Treatment Field Installation
Four SENTRE end treatments were installed, one at each corner of a single
bridge, as part of ADOT construction project HES-02202(33)P. This bridge is located at
approximately milepost 138.0 on US 60, the Wickenburg-Phoenix Highway, and
crosses the Beardsley Canal. The posted speed limit at this location is 55 mph, and
the site is in ADOT District 3. The project involved removing existing concrete curb,
and replacing it with a new concrete cub supporting a tubular thrie beam bridge rail
assembly. The original 40' width of clear roadway, consisting of two 12' lanes and two
8' shoulders, was maintained. The highway is flat in the vicinity of the project and the
view is unobstructed. Figure 7 is a photo of the project site before construction.
The 1989 design ADT for this two-lane highway is 8,309 vehicles5. Sixty-two
percent of these are passenger type vehicles5. During the sixteen year period before
the onset of this project (1973-1988) there were 13 accidents involving collisions with
the existing bridge rail6. Six of those were injury accidents. The number of accidents
recorded divided by the number of years of the accident record yields a probability of
collision with the bridge rail of 0.813 for any one year. The corresponding probability of
at least one collision during the two year evaluation period is 0.965. These probabilities
are based solely on the number of previous incidents and no provision for changing
traffic conditions has been made.
The 1989 design ADT for this two-lane highway in the vicinity is 2,565 vehicles5.
Fifty-seven percent of these are passenger type vehicles5. During the sixteen year
period prior to the onset of this project (1973 to 1988) there were three reported
accidents at the canal bridge6. Based on this accident history the probability of an
accident at the canal bridge is 0.188 for any one year, and the corresponding
probability of at least one accident during the two year evaluation period is 0.340. The
probabilities presented here are based solely on the number of occurrences in the last
16 years and no provision for changing traffic conditions has been made.
CONSTRUCTION
Construction of the SENTRE system took place on December 19 and 21, 1988
and was performed by Klondyke Inc. The system was operational on December 21,
1988. The TREND system was installed on November 3, 1988 and was performed by
Nesbitt Contracting Co. The system was operational on November 3, 1988.
The construction procedures for the SENTRE and TREND were similar. The
forthcoming description applies to both systems unless otherwise stated. Plans for the
SENTRE and TREND projects are included as Appendix D and E of this report.
Procedure
A manufacturer's drawing of the assembly of post 2 is included as Figure 9 to
further aid in the description of the assembly of the SENTRE and TREND systems.
The SENTRE and TREND systems were constructed with concrete footings.
The concrete used was Class S concrete with fc= 4000 psi. The TREND footing is 21'
long, 4' wide, and 8" deep. The footing is increased to 3' deep in the 3' of the footing
furthermost from the bridge rail to accommodate the embedment of the redirecting
cable anchor. The SENTRE footing is the same except for its length. The SENTRE
footing is only 17.5' long as SENTRE only has five posts rather than TREND'S six.
The footings for the SENTRE system were poured on September 19, 1988 and
had a 28 day compressive strength of 4829 psi. The footings for the TREND system
were poured on October 17,1988 and had a 28 day compressive strength of 5839 psi.
The bottom plate of each of the slip bases is mounted on the concrete footings
with six 7.5" long, 314" diameter bolts. The SENTRE's five base plates were spaced at
36" on center and the TREND'S six base plates at 37.5" on center. The bolts were
epoxied into the holes with epoxy included as part of both the SENTRE and TREND
packages.
The upper plate of the slip base is mounted on the lower plate with four 2.5"
long 314" diameter bolts. This is illustrated in Figure 10.
The support posts of the SENTRE and TREND consist of 32 long W6.5~9 A36
steel posts with a slotted 112" thick steel plate welded to the end. The support posts
are bolted to the slip bases at these plates as shown in Figure 11. The bolts are
tightened to manufacturers specification of 60 ft-lbs torque.
Figure 11 Post Attached To Base Plate.
A 21" W6.5~9 steel blockout is attached to each post. The systems' thrie beam
panels are fixed to these blockouts. The panels are connected together by a
mushroom bolt in horizontal slots and overlap such that the overhang is away from the
free end of the system.
The three posts furthest from the bridge (posts labeled 1, 2, and 3 in figures 5
and 6) have sand containers attached. Posts 1 and 2 each support two containers,
both designed to hold 100 lbs. of sand. Post 3 supports two containers, each with 150
Ibs. of sand. The containers were filled with sand and the lids snapped shut.
A 23' steel redirecting cable goes through a hole in post 1 and is anchored at
the front of the system. The other end is fastened at a rear anchor location forming an
angle of approximately 25 degrees with the roadway. The cable is tightened to a
specified 100 ft-lbs torque. The cable and anchors are shown in Figures 12 and 13.
A 20' transition between the SENTRE and the tubular thrie beam bridge rail was
placed upon seven posts driven into the ground spaced at 3' O.C.. The posts are the
same type of post as used in the SENTRE system. The end panel of the SENTRE is
tied to the transition. As the transition is also of thrie beam configuration, it is important
to insure the concave component of the SENTRE and transition thrie beams are
concurrent. This special requirement was not foreseen in the construction of this
installation. The upstream thrie beam matched the SENTRE panels, but the
downstream splice was of opposite concavity to the SENTRE panels. The problem
required a special downstream splice. The special downstream splice and an upstream
splice are shown in Figures 14 and 15.
Figure 12 Front Anchor of Redirecting Cable.
Figure 13 Rear Anchor of ~edirectionC able.
.. -
Figure 14 Special Downstream Splice.
1 I
. An itemized list of the replaced components and associated costs is shown in
Table 1. The unit prices are those provided by EASl at the time of replacement.
DESCRIPTION UNIT PRICE QUANTITY COST
Post 1 $73.00 1 $73.00
Post 2 65.00 1 65.00
Post 3 50.00 1 50.00
Blockout 16.00 2 32.00
Nose 50.00 1 50.00
l00# Sand Box 35.00 2 70.00
Tax and Shipping 50.92
Labor 215.08
Total $606.00
Table 1 Itemized List of Replacement Costs (First SENTRE Hit).
The thrie beam fender panel was also damaged by this first impact, suffering a
slight crimp. Because of the minor amount of damaged sustained, and the substantial
cost of replacement ($227), it was decided that the crimp would be pounded out, as
opposed to replacing the panel.
Energy Absorption Systems, Inc. had claimed that only 48 hours were required
to receive replacement parts from the time of ordering. This was not the case. The
replacement parts of Table 1 were ordered on August 31, 1989 and not received by
ADOT until September 14, 1989. As a result, the attenuator was out of sewice for over.
three weeks.
According to the work report filed by District 3 Maintenance Division, the actual
repair process was simple and was carried out by three men working a total of eight
hours each. The total cost of the repair including parts, labor, shipping and tax came to
$606.
The second impact was discovered September 26, 1989. The Maricopa county
road construction project was still in progress on US 60, and the highway was still
temporarily realigned beginning just west of the SENTRE attenuators. As with the first
incident, there was a significant amount of heavy construction equipment and semi-truck
traffic. Two of the attenuators were discovered to be damaged; the southwest
more severely than the northwest. At the southwest attenuator, marks in the soil and
impact induced spalling of the SENTRE's concrete footing sewed as evidence that a
front end loader or scraper had scraped its bucket or blade all the way up to the
attenuator. The southwest attenuator was obviously impacted by a large construction
vehicle, possibly the same vehicle that had left the scrape marks in the soil. This
attenuator was impacted in the direction of travel and telescoping of the panels did
occur. The mushroom bolt attached to panel 1 was free to slide as designed. A
photograph of the damaged system is included as Figure 21.
Figure 21 Damaged Southwest SENTRE Attenuator.
The northwest attenuator was hit on the nose longitudinally. Post 1 was slightly
twisted but judged operational. An itemized list of the items replaced and associated
1989 costs is given in Table 2.
DESCRIPTION UNIT PRICE QUANTITY COST
Post 1 (SW) $73.00 1 $73.00
Post 2 (SW) 65.00 1 65.00
Blockout (SW) 16.00 2 32.00
Nose (SW) 50.00 1 50.00
Nose (NW) 50.00 1 50.00
100# Sand Cont. (SW) 35.00 2 70.00
Tax and Shipping 23.00
Labor ' 236.80
Total $599.80
Table 2 Itemized List of Replacement Costs (Second SENTRE Hit).
Once again EASl's claim of requiring only 48 hours from order to delivery of
replacement parts proved not to be the case. As a result the attenuator was out of
service for nearly a month.
The damaged TREND system was no longer parallel with the roadway, and it's
future performance was doubtful. As such, Energy Absorption Systems, Inc. was
contacted for replacement parts and the system was repaired. Table 3 shows an
itemized list of components replaced and associated costs from EASl at the time of
replacement.
DESCRIPTION UNIT PRICE QUANTITY COST
Post 1 $73.00 1 $73.00
Post 2 65.00 2 130.00
314" Bolt, 2.5" 0.42 4 1.68
314" Washer 0.25 4 1 .OO
314" Nut 0.47 4 1.88
Blockout 16.00 2 32.00
Nose 50.00 1 50.00
100# Sand Cont. 35.00 2 70.00
Tension Strap, 79" 1 1 .OO 1 11 .OO
Tax and Shipping 56.90
Labor 1 12.97
Total $540.41
Table 3 Itemized List of Components Replaced (TREND Hit).
The thrie beam fender panel was also slightly dented. The minor amount of
damage sustained combined with the substantial replacement cost ($227) resulted in a
decision not to replace the component.
As with the SENTRE system, EASl could not maintain their claim of only 48
hours order-to-delivery time. Parts ordered on February 9 were not received by ADOT
until March 3. As a result, the attenuator was out of service for 37 days.
The repair was performed by an ADOT maintenance crew. The crew
dismantled and removed the damaged components, and replaced them with the new
parts. The crew of three men worked 4 hours, for a total of 12 man-hours, to complete
the job. The labor cost was $1 13, bringing the total cost or repair including parts, labor,
tax, and shipping to $540.
The extent of the damage to the TREND caused by vandals was the
unfastening and removal of the plastic lids of the sand containers. This happened on
several occasions and was corrected by merely re-fastening the lid by the inspecting
maintenance personnel.
In-Service Performance
During the course of the evaluation period three of the SENTRE and one of the
TREND attenuators had been damaged severely enough to require repair. All of these
impacts appeared to have been the result of heavy construction equipment. Because
in each of the incidents the vehicle did not remain at the site, it is inappropriate to draw
any conclusions on the safety performance of the attenuators per NCHRP-230. Due to
the lack of any reports of the collisions, it is reasonable to assume that in all instances
the occupants of the vehicle were not injured.
Although the impacts did not lend themselves to a safety performance
evaluation of the SENTRE and TREND, in-service performance information of the
systems was provided. After each of the impacts, the spare parts required for repair
were ordered from Energy Absorption Systems, Inc. promptly. In every instance it took
within a range of two weeks to 37 days for the replacement parts to be received by
ADOT. EASl had claimed a maximum of 48 hours order-to-delivery time for
replacement parts.
In each case, the repair of the SENTRE and TREND systems was simple, and
completed within a day by a three man maintenance crew.
Future Conslderations
As a result of the repeated excessive delay in delivery of replacement
components for the SENTRE and TREND, the Arizona Transportation Research Center
(ATRC), working with EASl representatives, had recommended that ADOT stockpile
one set of specific spare attenuator parts at a cost of approximately $1079. An
itemized list of the parts recommended by the ATRC is shown in Table 4.
DESCRIPTION UNIT PRICE QUANTITY COST
Post 1 $73.00 2 $146.00
Post 2 65.00 2 130.00
Post 3 50.00 3 150.00
Blockout 16.00 6 96.00
Fender Panel 227.00 1 227.00
100# Sand Cont. 35.00 4 140.00
150# Sand Cont. 45.00 2 90.00
Nose 50.00 2 100.00
Total $1079.00
Table 4 ATRC Recommended Stockpiled Parts.
In May of 1990, ADOT District 3 Maintenance purchased the spare parts of
Table 4 for maintenance of the SENTRE system. The total cost after tax and delivery
came to $1 174. There have been no incidents requiring repair of the attenuator system
since the acquisition of the spare parts. ADOT District 2 thusfar has not purchased any
spare parts in advance for the TREND system.
A concern brought to light by this experimental project, and subsequent
stockpiling of parts, is the problem with the proliferation of numerous end treatments. A
number of different end treatments with non-interchangeable components will require
substantially more warehouse space than only one or two acceptable end treatments.
RECOMMENDATIONS
Based on the experiences of the field installations of SENTRE and TREND in
Arizona, the following recommendations are presented:
1. Agencies should stockpile replacement parts for highway
appurtenances that may require repair. The timely repair or replacement
of attenuators and other appurtenances is crucial from both a safety and
liability point of view.
2. Agencies should utilize similar appurtenances in geographical
proximity to one another. The reason for this is to expedite repair by
allowing stockpiled parts to be kept nearby, rather than at a central
location. If two or more types of appurtenances without interchangeable
parts are close together, the storage space and capital investment
needed for stockpiled parts will be significantly increased.
3. Due to the nature of the impacts encountered, the TREND should be
subject to further monitoring until such a time that the FHWA has
developed sufficient database to determine if the system should be
upgraded to operational. The SENTRE was upgraded to operational in
April, 1989.
REFERENCES
1. Michie, Jarvis D., "Recommended Procedures for the Safety Performance
Evaluation of Highway Appurtenances." NCHRP Report 230 (March 1981).
2. American Association of State Highway and Transportation Officials, "Guide for
Selecting, Locating, and Designing Traffic Barriers." (1977).
3. Energy Absorption Systems, Inc., "SENTRE (Safety Barrier End Treatment) NCHRP
230 Certification Report." (May 1983).
4. Energy Absorption Systems, Inc., "TREND (Transition End Treatment) NCHRP 230
Certification Report." (December 1985).
5. 1989 Traffic Design Data, ADOT Materials Pavement Services.
6. Accident Location Identification and Surveillance System (ALISS), ADOT Traffic
Studies Branch.
7. Construction Costs 1987, ADOT Contracts and Specifications.
8. Construction Costs 1988, ADOT Contracts and Specifications.
9. Lattin, Douglas J., "SENTRE and TREND Attenuator Field Installations,
Construction Report", Arizona Department of Transportation (February, 1990).
APPENDIX A - FHWA APPROVED WORKPLAN
AZ-8802 (SENTRE)
AZ-8803 (TREND)
WORKPIAN
1. Evaluate and document the site selection criteria, and design conditions. This will include
expected service requirements of the project and anticipated service life of the device.
The geometric alignment, device location, traffic volume, vehicle operating speeds and
mix, environmental conditions, and soil stratigraphy will be documented.
2. Perform a risk analysis and pre-installation safety evaluation. Traffic and accident data will
be obtained and analyzed for each instaliation for the period 2 years prior to award of the
construction project. An appropriate risk analysis program will be selected and used to
determine the probability for collision for each device. Local maintenance authorities/ DPS
officers will be interviewed to determine if any unique safety or environmental conditions
are prevalent.
3. Assign reporting procedures and responsibilities. The ATRC will develop field evaluation
forms for use by ADOT construction and maintenance personnel and DPS officers. The
frequency and content of reporting will be established by the ATRC in conjunction with the
participating personnel.
4. Monitor and document the construction of the devices. The as-built condition of each
device and roadway condition wiil be documented and an emphasis placed on verifying
that design goals were achieved. Roadway friction testing will be conducted to document
skid properties at the time of construction. Field construction/contractor personnel will be
interviewed for suggested design/ procedural changes and/or improvements.
5. Prepare a construction report documenting the design and construction of the devices.
A construction report will be prepared in accordance with ATRC procedures for reporting
of experimental projects and submitted to the FHWA within 120 days after construction of
the last device.
6. Monitor in-service performance.
* The in-service evaluation will be conducted for 2 years,
Monthly field inspections will be performed by ADOT maintenance forces to
record 'brush hits" and drive away collisions, damage to the appurtenance,
required repairs, routine maintenance, and evidence of near misses. The
availability of replacement parts, level of technical support by the supplier, and
total down lime of the appurtenance will be documented. Unique problems such
as vandalism or corrosion will be identified.
* Reported accidents will be investigated by the ATRC as required. Damage to
appurtenances will be documented and video taped. Accident reporting will be
performed using techniques of the National Accident Sampling System or other
acceptable procedures.
* Traffic volume and mix will be obtained annually.
* ATRC wiil perform scheduled inspections of the installation at 6 months. 1 year,
and 2 years after construction. Interviews with maintenance personnel and DPS
officers will be pe&ormed annually.
* Annual maintenance costs will be collected by the ATRC.
7. Evaluate in-sewice performance. A before and after evaluation will be performed which
evaluates the relative effectiveness of the appurtenances. Specific appurtenance
performance will be evaluated on the basis of three factors: structural adequacy, occupant
risk, and vehicle trajectory after collision. The evaluation criteria are as follows:
a) Structural Adequacy: Measure of geometrical, structural and dynamic properties
of an appurtenance to interact with a selected range of vehicle sizes and impact
conditions in a predictable and acceptable manner. Nonvehicie colllslon-type
forces such as wind are not included. Criteria:
Acceptable redirection of vehicle.
Controlled penetration of vehicle.
Controlled stopping of a selected range of vehicle sizes impacting the installations
at spectied conditions.
Detached elements, fragments, or other debris should not penetrate or show
potential for penetrating the passenger compartment, or present undue hazard to
other traffic.
b) Occupant Risk: Vehicle responses of acceleration and velocity changes. Criteria:
Vehicle remains upright during and after collision although moderate roll, pitching,
and yawing are acceptable.
Minimize velocity change in vehicle. Small cars at both low and high impact
speeds are the critical test.
Minimize vehicle velocity change prlor to occupant impact
c) Vehicle Trajectory: Criteria:
Vehicle trajectory and final stopping position should intrude a minimum distance, ff
at all, into adjacent or opposing traffic.
For longitudinal barrier terminals, vehicle trajectory behind the test article is
acceptable in theory.
The evaluation will determine if the design goals were achieved, identify special problems
that affected appurtenance performance, examine impact the devices exhibited on other
highway conditions, and document the initial cost and annualized maintenance cost.
8. Prepare a final report A final report detailing the efforts of this study and the conclusions
and recommendations will be prepared. This report will be prepared within 90 days of the
completion of the final evaluation in accordance with ATRC procedures for reporting
experimental projects.
APPENDIX B - CURRENT ADOT STANDARD DETAILS
APPENDIX C - EASl TECHNICAL DISCUSSION
TECHNICAL DISCUSSION
TEST CONDITIONS
Test F a c i l i t y
The Energy Absorption Sy s~~.nIsn c. t e s t f a c i l i t y i s located a t the Lincoln
Airport i n Lincoln, California. The test area i s situated on f u l l y asphalted
level ground and has been cleared of a l l obstructions f o r unrestricted t r a j e c t o r y
of the vehicle. The s o i l i s composed of very s t i f f to hard s i l t s and clays and
can be c l a s s i f i e d as a f4CHRP 230 type S-1.
Test A r t i c l e (Design)
The SENTRE (see Figure 0-1) has been designed and constructed to provide
structural adequacy minimum occupant r i s k and minimum vehicle t r a j e c t o r y as Set
7 )
f o r t h i n NCHRP 230 Table 3--. "Crash Test Conditions f o r Minimum Matrix".
(Table 1)
The SENTRE i s designed as an end treatment f o r w-beam or t h r i e beam guard-r
a i l which w i l l redirect the nose of the impacting vehicle away from the
unyielding guardrail while a t the same time dissipate the energy of the impacting
vehicle.
The SENTRE consists of f i v e nested overlapping t h r i e beam fender panels
which telescope rearward i n response to a longitudinal impact force and an angled
side cable f o r urging the f i r s t fender panel and post assembly l a t e r a l l y away
from the fixed guardrail end. The fender panels and angle side cable function to
direct the nose of the impacting vehicle away from the hard point on the
guardrail while at the same time dissipating the impact energy of the vehicle.
The fender panels are slotted and secured together i n a nested fashion by
fasteners which allow the fender panels to telescope upon the application of an
axial impact force. The fender panels are supported above the ground on v e r t i c a l
support posts which are positioned on s l i p bases. These s l i p bases allow the
port< t o break away from submerged ground anchors so t h a t the fender panels may
telescope.
The f i r s t fender panel, or more s p e c i f i c a l l y i t s v e r t i c a l support post i s
connected to a r e d i r e c t i o n i n g cable. This cable i s secured t o an anchor located
a t the f r o n t of the u n i t , and a rear anchor located a t a l a t e r a l p o s i t i o n away
" f r o m t h e g u a r d r a i l . This cable i s positioned so t h a t when a l o n g i t u d i n a l impact
force i s imposed on the f r o n t o f the system, the cable w i l l urge the f i r s t fender
panel l a t e r a l l y as i t telescopes rearward. The l a t e r a l force o f the cable and
f i r s t Post i n conjunction with l a t e r a l forces contributed by the subsequent posts
w i l l urge the vehicle away from the hard p o i n t on the g u a r d r a i l .
Test A r t i c l e (Construction)
The SENTRE drawings are shown i n Appendix 0. The f o l l o w i n g discussion w i l l
describe how the i n d i v i d u a l components are constructed and assembled i n t o a
working u n i t .
The 52 inch, I0 gauge t h r i e beam fender panels include 32 inch s l o t s and are
secured together by fasteners (mushroom b o l t assembly). These s l o t s allow the
fender panels t o telescope upon the a p p l i c a t i o n o f a l o n g i t u d i n a l force.
The mushroom b o l t assembly i s designed with a shoulder t h a t t r a v e l s i n the
s l o t o f the fender panel. The assembly secures two overlapping fender panels w i t h
a grade 5, f l a t head 3" x 5/8" diameter b o l t which passes through a hole i n the
center o f the mushroom washer and a hole i n the underlying fender panel. The
mushroom b o l t assembly i s constructed so t h a t it does not s o l i d l y clamp the two
fender panels together, but rather secures them i n a p o s i t i o n r e l a t i v e t o one
another with s u f f i c i e n t tolerance t o allow the f i r s t fender panel t o telescope
i n t o the second panel. The l o n g i t u d i n a l movement o f the f i r s t fender panel i s
halted when it reaches the end o f the s l o t .
The fender panels are supported above the ground by v e r t i c a l support posts.
The 32" l o n g pos t s a r e W 6.5 x 9 s t e e l "I"be ams t o which an a d d i t i o n a l 21 i n c h W
6.5 x 9 "I" beam blockout i s bolted with two 1 1/2" x 3/4" diame'er b o l t s . The
fender panels are then attached t o the blockout with two 2" x 3/J" diameter grade
2 bolts. The purpose o f the blockout i s t o prevent automobiles w i t h small wheels
from snagging on the v e r t i c a l support posts o f the SENTRE during a side angle
impact.
The v e r t i c a l support posts are welded t o a 1/2" x 8" x l i ' steel s l i p base.
The s l i p base assembly includes a top p l a t e and a bottom p l a t e which are secured
t o each other. The bottom p l a t e i s attached t o an earth anchor.
The top and bottom s l i p base plates each include four open ended s l o t s which
are designed to receive 2" x 3/4" diameter b o l t s which secure the plates
together. The plates are large enough so t h a t they w i l ' l not y i e l d during a
l a t e r a l impact. The s l o t s are open ended so t h a t when a s u f f i c i e n t l o n g i t u d i n a l
impact force i s applied t o the v e r t i c a l support post by the impacting vehicle,
the plates w i l l s l i d e apart. To insure t h a t the plates w i l l s l i p apart i n a
predictable manner, they are separated by four 3/4" diameter f l a t washers. The
washers provide a consistant bearing area between the two plates so t h a t the
force needed t o cause the plates to s l i d e can be c o n t r o l l e d . Testing has shown
t h a t t h e vehicle sustains acceleration l e v e l s o f 4 t o 5 "Gs" when a torque of 60
f t . - l b s . i s applied t o the four s l i p base bolts. 22
The v e r t i c a l suoport posts also include a 4" x 4" steel gusset attached from
the v e r t i c a l support post to the top of the s l i p base p l a t e . This gusset
strengthens the v e r t i c a l support p o s t d u r i n g r e d i r e c t i v e impacts.
An additional 3'' x 6" angle p l a t e i s welded to the bottom s l i p base t o
provide a ramp and prevent possible snagging on each other as they break away and
move rearward i n response t o a l o n g i t u d i n a l impact force.
The f i r s t v e r t i c a l support post i s s i m i l a r i n construction t o the other
posts except t h a t it contains a 1 3/4" x 2" diameter schedule 80 steel pipe
gromet. The gromnet i s located 1 1/2" from t h e t o p o f the s l i p base and i s
designed to receive a 1 1/2" diameter threaded steer f i t t i n g which i s swedged to
the end of a 7/8" diameter steel cable. The cable extends from the previously
. .
mentioned front cable anchor, through the gromnet, to the rear cable anchor.
The rear anchor is located on an imaginary line which runs through the center of
the f i r s t vertical post a t an angle of 25 degrees w i t h respect to the centerline
of the roadway. The cable forces the f i r s t fender panel and vertical post to
move l a t e r a l l y upon the application of a longitudinal impact force.
The front and rear cable anchors are typically embedded in a concrete
foundation measuring 18" diameter by 4 feet deep. The front and rear anchor
consist of a 1" x 3" x 29'' steel bar welded to a 1/2" x 5" x 7" plate. The
anchors are designed to be universal and secure each end of the cable. The front
cable anchor i s positioned ahead of the f i r s t vertical support post and secures
the clevis end of the cable using 1 5/8 " diameter pin and cotter pin. The cable
passes through the gromnet in the f i r s t vertical support post and i s then secured
to the rear cable anchor by inserting the threaded f i t t i n g on the end of the
cable through the 1 3/4" diameter hole in the steel anchor and attaching a washer
and nut. The I 1/2" nut i s torqued to approximately 100 f t . - l b s . The cable
aids in redirectioning vehicles which impact the SENTRE head on. By urging the
f i r s t fender panel l a t e r a l l y the cable imposes a lateral force on the fender
panels. The cable i s constructed from 7/8". 6 x 25 IWRC. galvanized. steel cable
and will stretch 1 to 1 1/2 % of i t s length upon application of a longitudinal
impact force.
The lateral force will now be described in more detail. When a vehicle
impacts the guardrail end terminal head on. the f i r s t panel i s forced backwards
telescoping into the second panel. As the vehicle continues i t s motion, the
f i r s t vertical post impacts a second vertical support post causing the top plate
of the second s l i p base to disengage. The rearward movement of the f i r s t panel
stretches the cable until the cable will not stretch any further. The cable then
urges the f i r s t panel l a t e r a l l y causing the f i r s t fender panel t o give a smal 1
l a t e r a l impulse t o the nose o f the impacting vehicle. As the f i r s t fender panel
. .
reaches the end o f i t s t r a v e l the second fender panel begins t o telescope i n t o
the t h i r d fender panel. The f i r s t fender panel reaches the end Of i t s
- , l o n g i t u d i n a l movement before the second s l i p base breaks free. Each s l i p base
.decelerates and dissipates some of the energy of the impacting vehicle. This
process continues u n t i l a l l the s l i p bases of the SENTRE have disengaged g i v i n g a
large l a t e r a l force t o the impacting vehicle. The net consequence o f t h i s l a t e r a l
force moves'the vehicle away from the hard point.
The SENTRE includes additional mass i n the form o f sand because the s l i p
bases do not remove a s u f f i c i e n t amount o f energy t o keep an impacting vehicle
from h i t t i n g the hard point. The sand i s held i n containers which add 200 l b s .
t o the f i r s t and second v e r t f c a l support posts a t a 24 i n . center o f g r a v i t y and
300 lbs. t o the t h i r d v e r t i c a l support post a t a 21 in. center o f g r a v i t y . The
200 lbs. o f sand i s e q u a l l y d i v i d e d i n t o two 100 l b . containers f o r each of the
f i r s t two posts. A 100 lb. container i s placed on each side o f the block out and
v e r t i c a l post so t h a t two 2" x 3/8" diameter b o l t s can be inserted and clamp the
containers t o the v e r t i c a l post assembly. The 300 l b . sand mass i s equally
divided i n t o two 150 lb. sand contaners and attached t o the t h i r d post i n the
same manner as the 100 l b . sand containers. A l i d i s included on each container
t o keep moisture from entering the sand. The l i d i s designed w i t h a s e l f l o c k i n g
feature so no assembly tools such as r i v e t s are required.
A b u l l nose has been included as p a r t o f the design t o aid i n the aesthetic
appeal. The b u l l nose i s made o f gray p l a s t i c and includes a f l a t section on the
nose which may be used t o attach r e f l e c t i v e markers.
The SENTRE may be attached t o e i t h e r w-beam or t h r i e beam g u a r d r a i l . Both
guardrail types must include an end anchoring system which extends from a ground
anchor t o the section o f guardrail imnediately f o l l o w i n g the SENTRE. A
t r a n s i t i o n panel must be included when i n s t a l l i n g the SENTRE on w-beam g u a r d r a i l .
The t r a n s i t i o n panel i s connected from the l a s t fender panel o f the SENTRE t o :he
. .
hard p o i n t of the length o f need g u a r d r a i l . A 7/8" 6 x 19 IWRC cable extends from
a concrete deadman, located betweeen posts 4 and 5, t o a cable anchor on. the
t r a n s i t i o n panel. The cable and concrete deadman are strong enough so t h a t the
L.O.N. guardrail deveIops i t s f u l l t e n s i l e strength d u r i n g a r e d f r e c t impact.
APPENDIX D - CONSTRUCTION PLANS FOR AZ-8802 (SENTRE)
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APPENDIX E -CONSTRUCTION PLANS FOR AZ-8803 (TREND)